Extent of Relevant Hematopoietic Diseases, and Blood/Bone Marrow Transplantation. Currently, bone marrow, cytokine-mobilized peripheral blood, and cord blood are used as sources of hematopoietic cells for both autologous (same-individual) and allogeneic (histocompatibility-matched individuals) transplant. The availability of purified populations of functional hematopoietic cells is important to both types of procedure. The use of bone marrow transplantation in the United States has grown from a few individuals in the late 1960s, to over 10,000 per year in the United States, and over 6,000 in Europe..sup.20-22 Bone marrow transplantation is performed for a variety of reasons. Autologous transplants are used primarily as a salvage procedure in which blood or bone marrow is taken and stored prior to an intensification of radiation or chemotherapy. This procedure is rapidly becoming an important adjuvant for the treatment of breast cancer, and other solid tumors. Autologous transplants also are used to treat hematological disorders such as lymphomas, and leukemia, although differences exist in the primary indication for this treatment modality between the U.S. and Europe..sup.20 Likewise, allogeneic blood or bone marrow transplantation are used in the treatment of hematological malignancies, aplastic anemia, a number of congenital disorders, and immunodeficiency disorders..sup.21
Although the success of blood and bone marrow transplantation is undisputed, many difficulties still exist with the procedure, and, unfortunately, a number of patients cannot have, or do not survive, the procedure. Three major risk factors affect the outcome of bone marrow transplantation: purging of contaminating cancer cells, Graft Versus Host Disease (GVHD), and donor availability. Each of these can be reduced by the use of the functionally defined population of cells described above. In the case of autologous transplant, one of the mechanisms of disease-relapse is that malignant cells are infused with the hematopoietic cells. Thus, transplant cell preparations must be manipulated (i.e., purged) to remove malignant cells. The most effective way to do this is to positively select only the (functional) hematopoietic cells, therefor discarding the negative (cancer) cells. The use of the isolated functional cells described above improves the selection of hematopoietic cells by a factor of 10, thus greatly improving selection against contaminating malignant cells. In allogeneic transplants a major problem is that the immune cells in the donor marrow (i.e., the graft) immunologically reject the recipient's (i.e., the host) tissues. Thus, it is important in allogeneic to give as few donor cells as possible in order to prevent GVHD. Again, the use of lectin-positive cells will allow a 10-fold reduction in the number of cells given. These cells also might improve donor availability as the reduction in GVHD incidence could allow the use of donors with minor histo-compatibility mis-matches. Finally, the ability to purify functional hematopoietic cells will provide an excellent target cell population for gene therapy approaches to treating molecular diseases.
Hematopoiesis, the process by which mature blood cells are produced throughout the lifespan of the individual (producing on the order of 10.sup.11 -10.sup.12 blood cells per day) is tightly controlled. During blood cell development, cell:cell interactions, hematopoietic growth factors, and other microenvironmental molecules all function in cohort to regulate the production of distinct blood cell lineages within the bone marrow. Hematopoietic cells express a remarkable variety of cell-surface structures that mediate these interactions.(1) Among these cell surface antigens, CD34, a 105-120 kDa glycoprotein, is expressed on the surface of human hematopoietic cells, and thus serves as an antigenic marker for their isolation and characterization.(2-4) CD34.sup.+ cell populations also contain cells capable of both short-term, and long-term in vivo hematopoietic reconstitution.(5) The purification of such subsets of human hematopoietic cells using CD34 and/or other surface-markers, therefore, is important for their utility in bone marrow transplantation, and for their usefulness in understanding human stem cell biology. The functional characterization of human hematopoietic cells is dependent on in vitro assays.(6) However, these assays show that not all CD34.sup.+ cells are functional in vitro, as only 10-20% of CD34.sup.+ cells proliferate to give rise to colonies, or clones of differentiated progeny.(7-9) Other antigenic markers are used in conjunction with CD34 to further isolate and/or characterize hematopoietic cells. For example, the expression of the HLA-DR histocompatibility antigen defines more mature hematopoietic progenitor cells, and its absence, more primitive cells; other antigens, such as CD 71, Thy-1 or CD 45Ra, also mark more primitive cells.(10-12) None of these antigens, however, are expressed solely on the functional subset of CD34.sup.+ cells, i.e., the proliferating and, hence, clonigenic cells. Therefore, the identification of a cell surface structure which solely identifies proliferating CD34.sup.+ cells would represent an effective means for the isolation of these cells for clinical or scientific purposes. We report that all proliferating CD34.sup.+ cells are found within a distinct subpopulation that expresses galactose-specific cell-surface lectins.